1. Field of the Invention
The present invention relates to a zoom lens system and an image pickup apparatus using the same.
2. Description of the Related Art
A digital camera is spreading which is configured to photograph a subject by use of an electronic image pickup element such as a CCD image sensor or a CMOS type image sensor instead of a silver halide film. In recent years, as such a digital camera, people like a small and thin type. A size of the camera in a thickness direction mainly depends on a size of an optical system. Therefore, to achieve miniaturization, a constitution of the optical system becomes important. In recent years, a so-called collapsible lens barrel has become popular in which the optical system is protruded from a camera body during photographing and in which the optical system is stored in the camera body during carrying. Therefore, a lens unit constituted in consideration of a size of the camera when collapsed is remarkably important for a zoom lens system. As a zoom lens system having a small number of lenses, as disclosed in Japanese Patent Application Laid-Open Nos. 2001-133687 and 11-119100, such a type that has, in order from an object side, a first lens unit having a positive power, a second lens unit having a negative power, a third lens unit having a positive power and a fourth lens unit having a positive power is known.
On the other hand, a zoom ratio of a compact type of digital camera is generally about three-fold, but a type having a higher zoom ratio is also demanded.
However, in Example 3 of Japanese Patent Application Laid-Open No. 2001-133687 described above, the number of the lenses is set to eight to miniaturize the camera, but the zoom ratio does not reach three-fold. In Japanese Patent Application Laid-Open No. 11-119100, the number of the lenses is set to seven in Example 1, and set to eight in Examples 3, 4, but similarly the zoom ratio does not reach three-fold.